[Primary Pulmonary Diffuse Large B-cell Lymphoma Mimics Advanced Lung Cancer:Report of a Case].

Primary pulmonary diffuse large B-cell lymphoma( DLBCL) is rare, accounting for 0.4% to 1.0% of all malignant lymphomas and 0.45% of all lung malignancies. We report a case of primary pulmonary DLBCL caused by methotrexate-associated lymphoproliferative disorder (MTX-LPD). A 73-year-old man was referred to our hospital due to a growing lung nodule. Transbronchoscopic biopsy did not confirm the diagnosis, but positron emission tomography-computed tomography (PET-CT) showed an accumulation of SUVmax 28.7 in the same area and SUVmax 40.5 in the contralateral mediastinum, suggesting an advanced primary lung cancer. A partial thoracoscopic left lower lobe resection was performed in our department. Histopathological examination revealed AE1/AE3 negative, CD20 and 79a positive, bcl-2 positive, and a diagnosis of primary lung DLBCL. MTX-LPD was suspected, and discontinuation of the drug resulted in subsequent shrinkage of the residual tumor. If the diagnosis cannot be made by transbronchoscopic biopsy of an expanding nodule shadow, aggressive surgical diagnosis should be considered.